Thanks for the question.
Maybe a misunderstanding here, but the Imperial Guard is already in.

The Imperial Guard was made up of all arms and fielded many different units. The unit you refer to is the Old Guard Grenadiers, the are in the mod as Grenadiers. Other units from the Imperial Guard are also in such as, the Chasseurs a Cheval, Grenadiers a Cheval and the Young Guard.

The Imperial Guard was an elite corps of Napoleons army and his bodyguard, an army in its own right, similar to Hitlers Waffen SS, Hope this helps.

Regards,

Lord De Moray