If the system you use requires donor numbers for each person, you can add them directly on a person's profile.
From the profile, select add donor #.
Enter the donor number associated with the person's name in your other system then Save.
The number will be added to the profile and before the person's name, so you can easily see who already has a donor number.
To edit or remove the number, select edit donor # and change or remove the number.
You can use that number to search for a donor in the People tab or when entering donations. Just enter # and the donor number to pull up the person.