That has only been possible for the last few months at most. Marriott didn't use to transfer to LATAM/LAN AFAIK, that was an airline transfer they "inherited" from SPG in the merger, Sep 18 onwards.;

Meanwhile, MultiPlus wasn't forced on us LATAM Pass members until a few months before that.

So perhaps the first question to ask is whether anyone has transferred from anywhere to MultiPlus yet? I don't know if transfers are going direct to MultiPlus (or for how long), or if they're going (or have been going until very recently) to LATAM Pass and then being automatically moved to MultiPlus by LATAM Pass.