Easiest way is to paste in your core list, either as CSV or a multi-select field. Next, define a single-line text field and enter the additional select options needed for the applicable records as a list of options separated by commas. Finally, define a formula field with the formula
{Multi-Select Field}&IF({AdditionalOptions},', '&{AdditionalOptions})
You can either reconfigure that field as a multi-select and delete your original multi-select and the {AdditionalOptions}
text field or copy/paste the values from that field into your original multi-select and delete the last two fields. [Edit: I would like to apologize for the preceding sentence-like object.]
I described something similar and provided a demonstration base in this reply to an earlier post.
Let me know if you have any problems!