I think one solution is delete all the users from user manager and after insert one by one.
That would probably be the easiest as long as you don't need to keep any of the existing usermanager users.
It's possible to send one command from .net to delete all the records?
I don't know about .net, but you can delete all users from the router by running this command in the console:
/tool user-manager user remove [find]
I need to make a routine in .net to synchronize our user's table with the users into user manager.
Does this need to continue to be synchronized as users are added/removed from .net db? Or do you only need to do this synchronization only one time?
If it's only one time, I would export an excel or text file list of all the users from .net, and then edit the file using find/replace to get it into the format of:
/tool user-manager user add name=userName customer=admin etc...
Then, you can copy/paste the code directly into the router and add the users.