Trying to construct ticket on EK
 

I would like to fly SIN - LON with a stopover in DEL (Delhi)

Ideally, I would want to run this all on EK but since there is no SIN-DEL flight.

I would have to fly DEL-DXB-LON-SIN on EK which I can.

How can I add SIN-DEL to the same ticket ? Any ways around this on partner airlines ?

You can fly SIN-DXB-LON return with stop in DXB and separate ticket to DEL. Or as you mention SIN-DEL but don't think there's an EK Partner servicing that route.