As you said using "powerline adapters" may be a solution. The problem is that some work, some don't, and there is no way to predict how it will work in any particular installation. Your first order of business is therefore to purchase adapters from a vendor with a reliable return policy.
Once installed, configure the Express to "connect using Ethernet" since you will be using its Ethernet port to connect to your router.
The following explains in greater detail.
Note Apple recommends the following in the above
If you have configured your Wi-Fi base stations in the past, it may be helpful to do a factory default reset of each Wi-Fi base station that will be part of the wireless extended network before you begin.
To answer your question, it is not strictly necessary, but it may be easier to start that way.
Understand I shoudl expect unexpected as this powerline adapter might vary in terms of performance installation by installation literally.
thanks for your answer greatly!!
one more question if you dont mind... would you recommend any other ways to strengthen wifi signals? Would I need to upgrade airport extreme,which is my main wifi router hooked up to the cable modem? mine is 4th generation but I heard 5th generation has stonger wifi signal and better coverage... would this help address my issues?
There have been significant improvements in power output from the fourth generation to the current version, the most dramatic of which are evident in the 5 GHz wide channel mode. My personal but completely unscientific test, which consists of walking outside holding a MacBook while counting steps until I cannot connect any more, appear to support that claim.
This means you should enjoy in better signal coverage in your particular installation but there are so many factors that affect this it is impossible to say for certain.