Will an external flash drive work like RAM

Memory & Storage are very different things, you can't really use one to replace the other.

Sadly, your system has soldered memory which can't be modified but your internal SSD flash drive can be replaced!

As to using external storage: The speed of the device and the speed of the connection has a great bearing on its usefulness.

So what to do here?

I would work on clearing off as much as you can from the internal SSD drive. You really need to have 1/3 of it free if its a smaller unit and if you have a 512 or larger unit you can leave 1/4 free. If you can't achieve this you might want to look at getting a larger internal SSD drive. The more free space the system can leverage for virtual RAM and application Paging on the internal SSD drive the better.

As to the external drive it will be holding your documents and other files. If you can swing it I would recommend getting a thunderbolt HD or SSD drive. Otherwise, a good USB3 drive will do.

That is the best you can do.