Hello WiTSend,
This issue seems to be more related to BES rather than EV. I found some forums and BB Articles describing the same issue but with Exchange ActiveSync:
http://btsc.webapps.blackberry.com/btsc/viewdocument.do?noCount=true&externalId=KB34326&sliceId=2&dialogID=89339&cmd=displayKC&docType=kc&isLoadPublishedVer=&stateId=89353&docTypeID=DT_SUPPORTISSUE_1_1&ViewedDocsListHelper=com.kanisa.apps.common.BaseViewedDocsListHelperImpl
Overview
When attempting to add an Exchange ActiveSync account with the same domain as a work account on the BlackBerry 10 smartphone, the following message is displayed:
This email account cannot be added because your company manages all accounts that connect to <company domain>.
Cause
To enhance security of the domain specified for use with BlackBerry Enterprise Service 10, it is no longer possible to add an Exchange ActiveSync account with the same domain as the work account. This allows the administrator to ensure all accounts for the specified domain are secured with the same IT policy created by the organization.
This feature has been implemented on BlackBerry 10 OS version 10.1.
Resolution
This is by design and operating as intended.
However, there are some forums stating that if you use a different domain, you can add the email account with IMAP, but you need to make some changes to the AD user accounts. Take a look at this forum:
http://forums.crackberry.com/bes-10-f321/cannot-add-work-address-another-exchange-address-807418/
Either way, you should contact Blackberry for more assistance.
I hope this helps.