Currently, no you can't do what you want.
The code assumes all channels mapped on a bouquet are from the same network and have the same options. This allows the services to all be mapped/unmapped together. However, each network can have their own options (map encrypted, channel offset, etc.) and you don't want the "unmap network" button to unmap services from the bouquet that were from a different network, even though no-one would probably toggle the tickboxes anyway. So, there are a few changes that would need to be made to track which services are from where, and ensure this didn't break some other "merge channel" code that exists.
I've looked at it a couple of times but have not come up with a satisfactory idea. Perhaps we can create a global bouquet of all services seen, regardless of network, but we need to consider how it would work in the UI/server.
In the meantime, you can map services from the services tab and this will merge across networks. However, I believe you have to periodically do the mapping to pick up new channels and periodically press the "delete unseen services" button to delete services that no longer exist. This isn't done automatically since some channels in some countries are only broadcast at certain times (say 9--12) and aren't advertised as being available at other times in the day so could accidentally be removed if cleanup were done automatically.