Adds <sSquadronName> to <sSobGroupName>. This function can only be used in ".level" files.

<sSquadronName>: the name of the squadron being added. Note: a name is assigned to a squadron when it is created with addSquadron. If multiple squadrons have been given the same name, only the latest created is added to the sobgroup.
<sSobGroupName>: the name of the sobgroup. Note: it must be created beforehand with createSOBGroup.