You will need to re-encode it from VCD (MPEG1 form) to a SVCD (MPEG2 form). Just load the vcd mpg files into TMPGEnc, select the SVCD Template and set to CBR at about 2000 bitrate. Set to Highest Quality (very slow) and encode.
You could try the dirty trick of demuxing the VCD (with TMPGEnc) and then re-mux together but set at SuperVCD. Then burn as a non-compliant SVCD with Nero. No guarentee it will work, and waht you end up with is neither a VCD nor a SVCD.