Can I push a native 100G DWDM wave through a passive multiplexer?

We're currently feeding some smaller sites with 4-channel passive multiplexers since we haven't deployed active ROADM everywhere yet. Presently, these sites are fed with 3x10G DWDM waves over these 4-channel muxes. We'd like to upgrade to native 100G but we may be limited to what the mux can do. Ideally, we'd stand up the 100G wave alongside the existing 3x10G before tearing them down. My question is, if I shoot a 100G DWDM optic over that 4th channel, will it work? Will there be too much insertion loss with the existing 3 channels?
~Thanks!~