VariableA <- c("TRS34", "MMH22", "GFSR104", "GS23", "RRTM55", "P3", "S4")
gsub('(S)([0-9])','\\1-\\2',VariableA)
# => [1] "TRS-34"  "MMH22"   "GFSR104" "GS-23"   "RRTM55"  "P3"      "S-4"