Though it's feasible that this could be done simply based on the math (no change in usable capacity, essentially convert all of the pieces of parity data stored on the drives into "normal" data), I seriously doubt that the H310 will support RAID level changes while the RAID set is not in a healthy state.
Why would you even want to do this? Simply to make the blinking lights all be green again? Attempting to change the RAID level while degraded would put a heavy workload on the two remaining drives, and greatly increase your risk of an additional drive failure, which would bring things down completely.
I'd recommend you do what you need to in order to obtain a replacement disk. In the mean time, if you don't have a backup of the data, you should get one immediately... and hopefully it won't be too late. Until you have a replacement drive in hand to rebuild to, you should leave the RAID5 in a degraded state.
Now if I'm wrong about your reasoning and you want to stick w/ just 2 drives for the long term... You should probably just get a full validated backup, recreate a new RAID0, and restore data to the new RAID... and keep regular backups going for when the RAID0 eventually fails on you.
Addition after your edit: That clarifies things greatly...
So the general rule-of-thumb for foreign configurations is that if the foreign state is on multiple drives and causes the RAID to be shown as offline, then you import. If it's on a single drive and the RAID is still "alive", you simply clear the foreign config and rebuild. In your situation, an import might get things back up, or it might still leave you with a failed RAID... or the import may just fail altogether. If you have a backup to restore from, I'd go ahead and try an import. If that doesn't get the data accessible, then just delete the RAID, create a new one, and restore from backup.