|You're looking for a 'coupler'. Normally these are a small plastic device about 1" square with two RJ-45 (or RJ--11 if it's for a phone line) connectors in it.|
They work fine and as long as you're not trying to extend the total cable length beyond the maximum limit of 100 m, then it will work just fine.
I've gone one sitting on my desk here at home that I made with two keystones and a 6" long piece of cable. I even tested it on my tester at work and it tested fine....including the SNR test.