Give the smoke grenade a targetname.
e.g
Code:
new String:targ[64];
Format(targ, sizeof(targ), "nade%i", target_propIndex);
DispatchKeyValue(target_propIndex, "targetname", targ);
Then you need to attach it to that targetname
Code:
AcceptEntityInput(target_flareIndex, "SetParent", targ);