You can choose source levels for each source in the Source Selector of the Zone, but the IP1 wouldn’t control them individually. The Source Select shift function is purely to activate one source at a time.
A workaround to achieve what you want is to program 4 Presets that only change the IP1 settings. In each preset, configure the IP1 to control the crosspoint you need (Mic1 to Zone1, Mic2 to Zone1 etc), and the Shift function to select Preset.
This is a common feature request that we might add in a later release, but I cannot commit to a release version or ETA.