Formula to append field value into a different field that also allows user entry?

Is there a way to append a value from field A into field B that is a user entry field, while allowing user to also manually enter a value in field B?

We have a form to upload Documents that also captures information from another table (Task table). The process is that we create a Task record which includes a section to upload a Document - the Add Document link in the Task record opens a new Document record, and info from the Task autopopulates into the Document record.

One of the Task fields that autopopulates in the Document record is "Evidence Number" - (eg: 12A, 07B, 34D).

In the Document record, users create a name for the record in the field "Document Record Name" (eg: Server Log, New Hire Population)

We need the Evidence Number value listed in the Document Record Name and currently ask users to manually enter it. But its often left out, so we'd like the Evidence Number to autopopulate into Document Record Name, but still allow user to add their own "name".

EXAMPLE: if Evidence Number is 12A, then Document Record Name would autopopulate with "12A" or "12A -"but still allow user to enter values. After user enters "Server Log", the field value will appear as "12A Server Log" or "12A - Server Log".

Is this possible if we make Document Record Name a formula field? If so, what would be the formula? Also, would there be any issues with reports showing both values in the field? Thank you for your help!