Add my Domain to 365 without email

fedcap 

Hi, you can certainly add your custom domain into Office 365 without affecting your other email system yes.

As long as you don't change your MX records or Autodiscover records you will be fine.  Just add the domain in,  verify with a TXT record and you will be able to set your users addresses to yourdomain.com as the primary UPN suffix as opposed to tenantname.onmicrosoft.com.  

Just be careful if you are assigning licences to the users that include an Exchange Online licence as this will create a cloud mailbox.  This in itself will not affect your existing email routing but if you ever come to the point later down the line where you want to migrate your mail into Exchange Online, then having duplicate mailboxes will cause you extra hassles at that point.  You can exclude Exchange Online from the licence assignment to avoid this.  This can be done manually, or by group based licencing.