Adding a turning block at the mast base will not give you more purchase advantage.
Get your binoculars out and see if you can see the unused sheave next to the one that has a halyard.... or scoot up there yourself and have a look see. If you're talking about another jib halyard (one facing the fore deck) then it is likely your Catalina 30 has a masthead similar to this C270 drawing on the right. Mine doesn't have the boom topping lift sheave next to the main halyard... but it could be easily added. There are two jib halyards sheaves, as you can see. So it behooves you to inspect your boat more closely. One thing you might look for is an exit block for the missing halyard..... that would be a good clue.
BTW, a foredeck topping lift is a handy addition for lifting stuff around the foredeck, holding up whisker or spinnaker poles, launching and retrieving your tender...... I would just run it externally. Attach a "mast hound" or "eye strap"
somewhere between spreaders and masthead then clip on the appropriate block (don't use a pulley, they aren't marine quality). If you want to add 2:1 purchase, use a single block w/becket up top and a single block and shackle at the load end. A horn cleat on the mast to secure and store the tail.... the line runs up to the single w/becket then down to the single then back up to the becket where it terminates. The load clips to the single block.