Need to create claims with manual numbering

There are claims created in Legacy system which are synced with High Radius which is third party application for reconciliation, collections and cash applications.

We are considering to migrate open claims from this Legacy system for which we are making use of webservices but the same webservice does not allow claim number to be entered manually to match with the claim from Legacy system same is with creating it from UI.

There is no option to enter a claim number manually. Please suggest if there is any solution or a workaround for this.