Can I extend my sump pump outlet?
You should not add an upward bend to the pipe, as that will more likely lead to water always sitting in the pipe. 

Instead you should add a check valve (one way valve) to the line. This will allow the pump to push water out, while preventing ditch water from flowing back in. 

If the pump is strong enough, it should be able to push water into the ditch even when it's full. Unless the outlet is pointed upstream, in which case you should redirect the pipe downstream.