I completely agree with saywhatnot: "You can do the backdoor but you will have to consider your existing ira tax implications"
You need to roll existing pre-tax IRAs, that you do not want to convert to a Roth, into your new employer's 401k. Otherwise, you subject them to taxation. Not all 401k plans allow incoming rollovers, so check with your employer or, even better, check with your employer's 401k administrator as most HR departments are clueless when it comes to 401k plan details.