Precisely. The compressor must come in the signal chain before coming into the porta.
As for during mixdown, many think it is an over-used and unnecessary step in the chain, but you didn't ask for opinions. Line Out jacks to a two ch compressor input; Comp output to mixdown deck or other medium. Very gentle compressor settings maybe 1.5:1 or less.
If you had an outboard mixer or another Porta your options would be better.