I heard that hardware wallet can be imported into metamask to use some other network that aren't available on the hardware wallet, is this true? Also..
1. Can I import a hardware wallet into a mobile wallet like metamask?
2. What are the risks involved in doing this?
This is not recommended and I would not do that. A better solution is to create a secondary wallet with Metamask and use it separately from the hardware wallet.
You probably don't need all the coins available on Metamask wallet all the time. Why not keep your main stash on a hardware wallet where it's safe, and transfer only what you need to Metamask?