Can I add 1095-C to return already e-filed?

No.  You don't need to.  You don't enter a 1095-B or 1095-C on your tax return. You only enter a 1095-A if you get one. 

The only 1095 that must be reported is the 1095-A.  Both the 1095-B and 1095-C do not have to be included on your tax return.

Form 1095-C will indicate your name and the name of your large employer, the months during the prior calendar year when you were eligible for coverage, and the cost of the cheapest monthly premium you could have paid for coverage under your employer's health plan.  This also does not have to be included in your tax return.

 If this does not completely answer your question, please contact us again and provide some additional details.