Haha @AlanRynne, no screaming necessary 
That’s tricky unfortunately. I remember @Baris & the team at Ekkodale did something similar, but unsure how they achieved it. If you have direct control over the database - as it seems you do - it might be easiest to insert a record in the speckle db directly from your app for the respective user.
The currently supported way is to register your application on the speckle server as a third party app (or first party, if you control the deployment) and thereafter request the user’s permission for your app do to various things on their behalf (as defined through the scopes that you specifiy when you register your app). Once you have that, you can definitively tie in their account with your internal account!