You can't trick LT into running add-ons from 3rd party authors who used full AutoCAD to program and create their content: if you need 3rd party add-ons, you should not be using LT.
LISP is not an option in LT: outside of Australia it's a license violation to try and add that ability back into LT. As in Autodesk sues you, takes you to court, charges you $$$$$, then goes out of it's way to notify your professional associations and competitors in your region that they snagged you violating your software license. They also tell all other software vendors about you, who then may or may not come knocking to find out more. It's a pretty vicious process to say the least.
FYI there are no "upgrades" at Autodesk anymore: you region of the world may be running special pricing, contact a reseller and find out if you qualify for any discounts out there.
Good luck.