Sebastian played for Carmarthen Quins in Wales, before being called up to play for Wales Under-16s and Under-18s, which resulted in him joining the Scarlets Academy.
The prop has been at the Scarlets, who were drawn in the same European Champions Cup pool as Glasgow on Wednesday, for two years and featured for them in the LV Cup against Sale Sharks last season.
He is Scottish qualified through his father, who was born and raised in Edinburgh.
Sebastian said: “Joining Glasgow Warriors will be a great opportunity for me to impress, as well as to grow and develop as a player.
“They are Guinness Pro12 champions so it’s an amazing club to join. I’m sure they’ll teach me a lot and the move will have a positive impact on my career.
“The Glasgow team is filled with young, talented rugby players working in a great environment which I can’t wait to be a part of.”
Hunter, who joined the club at the start of last term, suffered a recurrence of an anterior cruciate ligament knee problem in pre-season, so missed the whole campaign. But he is now back to full fitness and training with the squad.
He was in the Glasgow Warriors squad which won the Melrose Sevens in April.
The 23-year-old, who has made 16 appearances for Edinburgh, came through the Scotland age-grade ranks. In 2012, he was awarded the Macphail Scholarship and spent four months playing club rugby in New Zealand.
Hunter said: “It was frustrating to miss last season, but I’ve been back training for a while and I’m enjoying getting the chance to show what I can do at pre-season training.
“The club have really looked after me during my time out and I want to repay them for the faith they’ve shown.”
Hill has been training with the Warriors since February. The 25-year-old played for the Scotland Club XV earlier this year, scoring a try and being named man of the match in a 23-18 victory over the Ireland Club XV.
The forward recently helped Heriot’s on their way to being crowned BT Premiership champions with a win over Melrose at Goldenacre.
Hill said: “I’ve really enjoyed my time training with the club and it’s made me hungry for more.
“I feel in good shape at the moment and I’m ready to take my career to the next level.
“Earning my first professional contract is a great achievement and I’m eager to get started.”
Bordill is currently in his third year at the club, after joining from Sale Sharks.
The flanker signed a partnership contract at the start of this season, after graduating from the BT Sport Scottish Rugby Academy Stage 3 programme.
The 21-year-old has played seven times for the Warriors and was also in the squad which won the Melrose Sevens. Bordill said: “Glasgow Warriors is a great club and I’ve really improved since moving up from Sale Sharks.
“I enjoyed playing a few games last season and with the Rugby World Cup on in September, it’s a good opportunity for me to get more game time.”
Coach Gregor Townsend is working on making more additions to the squad, with announcements expected as the summer progresses.
He said yesterday: “The strength and depth within our squad is something we really value and to have Javan, Jason, Will and Gregor with us next season is great news.
“With the Rugby World Cup being played at the start of the season, we know we’re going to have a number of players involved in that tournament and it’s important we have others who can step up.
“Javan is a mobile, young prop who has a big future in the game if he works hard, while we’ve been impressed with Jason when he has trained with us and in his performances for Heriot’s.
“Will has shown a real determination to improve and did very well when given the opportunity to play last season, so we are excited to see him progress further over the next few months. Gregor has had a tough time with injury but is now back to full fitness and has been making a very good impression in training.
“All four will make a positive impact at the club next season.”