You can import prefixed tags against supporters, but it's important to be aware that this process always creates *new* tags (it does not locate/apply existing tags).
To do this, simply add a TAGS column to your import file. In that column, for each supporter record, list the tags you want applied, separated by a comma. Here's an example of how to set up a spreadsheet which will apply 'interest:dogs' and 'interest:cats' to supporter key '123456':
The important thing to remember is that, if 'interest:dogs' (for example) already exists as a tag in your system, then the import will create another, identical tag with that name.
After setting up the import file, you can import the Tags column by selecting 'Multiple Tag Generator' against it in the 'Field Matching' portion of the import process.
Internal Reference: CL-470