Can I Pass a return value of a getter method to a setter method of another object in a different class?

Yes, you can - but not like that. You call the setter method, passing in the value as an argument - you're currently trying to assign a value to the method call, which makes no sense.

You want:

customer1.setExcursion1(cust_dest.get_excursion1());

(Your naming is inconsistent, by the way. It should be getExcursion1, at least - and ideally just getExcursion. What is the 1 meant to be for? Likewise cust_dest should be custDest if you're not changing the words - or ideally something rather more descriptive.)