Are your halyards external now? ie does the main cross both sheaves and come down the front of the mast and the jib the opposite? If so you could free up a sheave for a TL by making at least the jib halyard internal..
Otherwise if you've got a web or a strap that could take a block that should be fine. If you intend to hoist such loads, though, your topping lift will have to be beefier than normal.
Another alternative adjustment is to put a short 3-4 part tackle on the last few feet of the fixed TL and adjust its length there, at the boom end.. the bitter end could be run along the boom so as to be reachable from the boat at all times.
This diagram from the W32 site..