OMV3 is currently using php 5.6, but can we upgrade it to php 7?
Nope.
Any issue?
Lots of issues. First is that all php5-fpm configs and php files throughout OMV are hard-coded and/or file named for php5. Second issue is that the php5-pam library doesn't work with php7. Volker has a workaround using python but it didn't work on my system. Last, any update to the omv package would break everything again.