The 30-year-old McCusker was released by London Irish at the end of last season and despite a number of clubs chasing his signature, the back row has returned to West Wales having previously played over 70 games for the Scarlets.
The North Walian gained six Wales caps during his Scarlets career and captained the region for the 2012-13 season.
“Rob’s presence and professional background will have a positive impact both on and off the field and he is a welcome boost to the squad who are looking to improve significantly on last year’s league position,” said Thomas.
Tighthead prop Javan Sebastian makes a welcome return from Glasgow Warriors while the Quins have also snapped up the services of Wales Under 20 and former Scarlets Academy hooker, Torin Myhill, the brother of Cardiff Blues player Kirby Myhill.
“Torin is an excellent prospect and will bring strong competition for the position of first choice hooker,” said Thomas.
Another young prospect captured is Steffan Marshall from Llandovery who is a left footed outside half who Thomas describes as ‘a young player with heaps of potential’.
Longstanding Quins stalwart Haydn Pugh will be this season’s club captain for this season following Ian Jones retirement after five years of captaincy. Centre Rhys Williams will be vice-captain.
