[I] want to buy a swag plug-in chandelier but it only comes in a pendant lamp to be hung from the electric box in the ceiling. Can I change the electric cord with one that has a plug on the end to plug into the wall? I will need more chain and a long cord with a plug. Can I do this and how difficult would it be

What you need is Swag Kit, comes with chain, cord and plug, hooks, etc.

Westinghouse Lighting Swag Lamp Kit - Tool Catalog - General Purpose Hand Tools - Chisels (http://www.sears.com/westinghouse-lighting-swag-lamp-kit/p-SPM2676546502P?prdNo=2&blockNo=2&blockType=G2)