This question comes up often. The basic answer is it is much less
expensive to sell the GLT and buy a turbo model. The turbo engines
include different pistons, smaller diameter engine bore, sodium
filled valves, oil spray for piston bottom, lower compression ratio,
different ECU, different intake plumbing, an intercooler, a bunch of
check valves, etc. It all adds up to quite a bit of money.

A company name hosselman (or similar, I know that spelling is wrong, it
is a german company) used to sell a bolt on kit for 850s. I
think they quite making them since it was probably still cheaper to
sell a non-turbo and buy a turbo model then use the kit.