Outputs? Inputs? Wha???
First, let's get the terminology and signal flow straight. You need an OUTPUT from your receiver to the subwoofer's INPUT.
Are you quite sure the receiver doesn't have an "LFE" output?
If your receiver has preamp outputs, you have a fighting chance if your new subwoofer has a right and left rca jack type inputs. You can use this option.
If your receiver lacks these, then your subwoofer must have a pair of speaker level inputs.
If all the subwoofer has is one lonely RCA jack for an input, you're screwed. If your receiver don't have preamp outs and the subwoofer doesn't have speaker level inputs, you're equally screwed.
If these aren't possibilities, let's hope you can exchange that subwoofer for something you can use.