Not by the basic rules no, but the mini-campaigns are nice for trying out some house rules since they are a smaller slice of the game.
You could agree to give him Belethir on some mission or start with some amount of threat at the beginning of the campaign. Beware of balance problems though, maybe give the heroes something as well to even things out. Fortune alone probably won't do it in a mini campaign.
But if the overlord just want's to have Belethir on the field cause he wants the boost, then keep his power-lust in check.
Also, I know Bolgreth appears in the mini-campaign for Trollfens, so he'll still get a cool boss monster to play with.
Edit for rules reference:
On page one of the Lieutenant rules under "Plot"
Important: The overlord player can only use Plot and Agent cards when
playing a campaign.
You could argue that a mini-campaign is a
campaign, but that's your only stretch.
Edited by Carbini, 04 August 2014 - 02:35 PM.