I apologise if this feature has already been requested. It would be incredibly useful to assign characters that I, as DM, have created to campaigns. My current workaround is to recreate the character as an unassigned character in the campaign. 

You can create unassigned characters in a campaign that players can then claim, is that what you're looking for?

You can also add characters to a campaign that you have already created using the same join link you give to your players.