Was wondering if it's possible to transfer some custom patches from my s90 to modx?

No. Sorry... The S90 was 2002 (two decades is beyond the limit for those types of sounds - AWM2).

When you say custom patches (?) can you be more specific - the S90 could not load any new Waveforms, so the Voices were made with data from the factory Wave ROM.
So likely updated versions of most of the sounds of the S90 are represented in the MODX. The S90 was equivalent to the original Motif (less the ability to Sample and the record Sequencer).

While it might be possible to convert S90 sound to Motif sounds, then convert Motif sounds to Motif ES and Motif ES sounds to Motif XS - (the MODX can translate Motif XS sounds directly)... the universal disclaimer that goes: "... your results might not sound exactly the same." will be in full effect (lol). Consider that not only might a substitute waveform be used, but the envelopes, filters, effects, etc., etc., are now so much better, it's pretty much a guarantee it will not sound exactly the same.

We recommend you jump into the MODX and create some new "custom patches" - you have a much, much improved tool to do so.