How to exert force on a component sphere element of a multisphere clump?

You can use compute rigid property fcm to access the force of every clump at the center of mass

#compute clump properties
compute bid all rigid property id_multisphere #ID
compute btype all rigid property clumptype #Type
compute xcm all rigid property xcm #center of mass
compute fcm all rigid property fcm #force at center of mass
compute quat all rigid property quat #rotation arround COM
compute vel all rigid property vcm #velocity of COM
compute sc all rigid property masstotal #total mass of clump
compute cs all reduce sum c_sc #mass of all clumps in system
#
dump rigids all local ${dumpstep} post/rigids*.dump c_bid c_btype c_xcm[1] c_xcm[2] c_xcm[3] c_quat[1] c_quat[2] c_quat[3] c_quat[4] c_vel[1] c_vel[2] c_vel[3] c_fcm[1] c_fcm[2] c_fcm[3] c_sc

Than you can load the dumpfile in ParaView and apply a threashold filter by clump-id (c_bid) to extract only one clump, and than plot the force over time with "PlotSelectionOverTime" Filter.

To enable set and group for clumps one can use an undocumented flag for fix-multisphere:

fix 2 clumps multisphere allow_group_and_set yes #integrator for clumps

But be carefull, the variable-group function (mass,fcm..) are wrong.