You'd use the converter "Tag - Tag" for that. It allows for copying information from one field to another. To copy the Artist to the Album Artist, you'd use
Copy Artist to Album Artist (replace existing data)
Field: ALBUMARTIST
Format string: %artist%
To preserve what's already there and simply append the Artist:
Append Artist to Album Artist
Field: ALBUMARTIST
Format string: %albumartist% %artist%
Further reading would be using an action "Format value" and only fill in the Album Artist if it's empty