You can combine (i.e. merge) several user database files (extension *.mtu) into a single new one. In order to do so, please press the "Combine" button at the top of the User Database Manager, mark the mtu-files you would like to combine in the "File/Open" dialog that is displayed, and press "Open".
Afterwards, you will be asked about the directory and file name for the target mtu-file, so please make the appropriate selection and press "Save". The merging of the files will now be carried out; a corresponding progress window will be displayed that will close automatically when the merging has finished.