Get the Imperial Assault Campaign module for Vassal from http://www.vassalengine.org/wiki/Module:Star_Wars:_Imperial_Assault
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in the Imperial deployment deck, there is one cheap card that allows the imperial player to deploy up to 2 Mercenary groups.
Note that this is for Skirmish (the Temporary Alliance skirmish upgrade card). The skirmish upgrade cards are not used in the campaign.
Like said above, in the campaign all figures controlled by the imperial player are imperial figures, whether their affiliation is imperial () or mercenary ().