Yes. You can use the SEND (1, 2) jacks as SUB OUT jacks. Select "SUB OUT" or "SUB OUT, AUX MUTE" for SEND 1/2 SETTING in HARDWARE SETTING.
[MENU] button > HARDWARE SETTING > OTHER
*When "SUB OUT" is selected, the RETURN jack works as an AUX IN jack. Note that the signal sent to SUB OUT will also be output from the OUTPUT jack if you do not connect a plug into the RETURN jack. For details, see "HARDWARE SETTING" > "OTHER" in the GT-1000CORE Parameter Guide.