Edinburgh professional Hogg has been out of action since before the Autumn Tests with a hip injury and he will augment the Myreside outfit at the Greenyards as they begin their bid to retain the Kings of the Sevens crown.
Watsonians playmaker Mike Ker said: "It is great to have someone like Ally available to us. He has never played at the Melrose Sevens before and he is, obviously, keen to get back to full fitness."
Ker and Co will take on either Currie or Hawick in round one and will be looking to go one better than 12 months ago when they were beaten at the semi-final stage by eventual winners, the University of Johannesburg.
The Watsonians squad sees Craig Sorbie come in for departed hooker Tupu Saena and former Scotland player Marcus Di Rollo in the centre.
Heriot's captain Graham Wilson will be playing in his third Melrose Sevens. Last year, the Goldenacre men reached the quarter-finals before losing out to eventual finalists Melrose 14-5. They are likely to have Scotland Sevens caps Struan Dewar and Chris Fusaro in his squad along with fit-again flyer Marc Teague.
Ahead of their opener against Glasgow Hawks in the preliminary round, Wilson said: "We have had a few bumps and bruises since last weekend's cup loss at Ayr but, hopefully, everyone is fit. To play at Melrose is a great honour."
Boroughmuir look set to have sevens specialist Charlie Keenan back in the fold for the first preliminary round tie of the day at 11.30am against Stewart's-Melville. The winner will take on Cambridge University.
Also in the preliminary round, Edinburgh Accies take on Kelso.
Draw: Stewart's-Melville v Boroughmuir (winners v Univ of Johannesburg); Hawick v Currie (winners v Watsonians); Heriot's v Glasgow Hawks (winners v Cambridge Univ); Kelso v Edinburgh Accies (winners v Ayr); Langholm v West of Scotland (winners v Army); Dundee HSFP v Peebles (winners v Melrose); Jed-Forest v Aberdeen GSFP (winners v Hamilton).