They could use a rule (a extended version of Pirazzo's Lost legion):
Mixed formation: The first rank of a Pike & Shotte unit is always made up of Handgunners, and all Handgunners must be placed in the first rank. The second and following ranks are made up entirely of Spearmen. During the game, remove casualties from the back as normal, it is assumed that the Spearmen step forward and discard their spears to pick up the Handguns of the fallen first rankers. Only when all the Spearmen have been killed, can casualties can be taken from the Handgunners in the first rank. You can choose to have a Marksman or a Sergeant as member of the Command Group.
Close Combat: if the unit is attacked from the front, we assume that the Handgunners withdraw behind the Spearmen. However, the unit can still declare a S&S reaction as normal: the Handgunners first fire their weapons, before they withdraw. The first rank of Spearmen counts as being in base to base contact with the enemy, and all attacks are made by and/or directed at the Spearmen, as long as there is one full rank of them. This does not apply if the unit is attacked from the flank or the rear.
That is not going to happen, of course.