Adding a squadmate to a level with this pack is not too hard.

Coalesced change

Add a coalesced entry like below to the biogame.  This will tell the selection GUI to add the squadmate to the squad selection.  There will only be a maximum of 2 squadmates available due to the limitations of the GUI how many it can show (9).

<Section name="sfxgamecontentdlc_mod_egm.sfxseqact_sethenchmissionavailability">
<Property name="missiondataarray">
<Value type="3">(MapName=BioP_Cit004,
MissionAvailability  =(
(HenchTag=hench_zaeed,Mandatory=FALSE,Optional=TRUE,Disabled=FALSE,ForceBonusSquadmate  =TRUE,PartyCheck=0),
(HenchTag=hench_kasumi,Mandatory=FALSE,Optional=TRUE,Disabled=FALSE,ForceBonusSquadmate  =TRUE,PartyCheck=175)
        )
       )
</Value>
</Property>
</Section>

MapName = BioP of the level
MissionAvailability add one per squadmate (if you do not add a squadmate they are unavailable by default).
HenchTag = the squadmate's tag

Each squadmate should have one of these options:
Mandatory = Preselect this squadmate, forcing the player to take them.
Optional = Allow this squadmate to be picked if the user wants.
Disabled = Grey out the squadmate, preventing them from being picked.

Then pick the 1 or 2 squadmates of the 9 you want to show by setting ForceBonusSquadmate=TRUE.  This will override systems like EGM where there are GUI options to pick availability from the nine.  

If there is a chance that the squadmate might be dead or not available you can add a PartyCheck.  This should point to a conditional in your mod that sets if the squadmate is available to the party (so if they are dead or not recruited it should be false). In the above example hench_kasumi checks that she is alive from ME2. If they should always show, then leave this as 0 or remove.


Dialogue changes

Every conversation and cutscene in the level must be adjusted so that even if the squadmate doesn't say anything, there is a dialogue path that allows the cutscene to continue if no vanilla squadmates are available.  Of course better to add actual dialogue, but this is the minimum. By default it should be always the vanilla squadmate dialogues first, so if one normal squadmate is taken on the mission, they will get all the lines.