Seems that the Hansa is the origin of some merc units itself. In fact one, namely Cynthia Hickock of RDF 3, was a merc, prior to her becoming Captain General. Also, FM Periphery says that the Hansa also brokered contracts for the states of Nueva Castille. I take this to mean that there is a home-grown merc industry in the Hansa - and why not, they are merchants after all.
Also note that the Hansa has actually incorporated mercs in its rosters which led to an improvement in their quality rating. Could it be that occasionally the Hansa uses parts of its own military forces as mercs?
As to mercs from the IS working with the Hansa, its quite credible. We are told that they travel long distances in the deep periphery to trade with the Jarnvolk, so why wouldn't they travel to the IS. Of course, only small, hard luck units would probably go there, but why shouldn't they? If even parts of the shattered Ice Hellions operated in that area as mercs, why wouldn't others? *BTW we know of one such unit, the Langford Wraiths, who operated in the area as well.