Sorry, you can't associate personal VSTS to an Organisation Although both identities use the same email address, they're still separate identities with different profiles, security settings, and permissions.
Please see the Q&A below:
Why must I choose between a "work or school account" and my "personal
account"?
- Choose Work or school account if you want to use your organization's directory to authenticate VSTS users and to control
VSTS account access. This limits access to members in your
organization's directory. All other users must sign in with work or
school accounts too.
- Choose Personal account if you want to use your Microsoft account with VSTS. All other users must sign in with Microsoft
accounts too.
- If you feel annoying about the pop-up, you could just follow the
link to rename your personal Microsoft account.
- If you are worrying about the subscribers:
Within the Visual Studio subscriber portal, you may be able to add an
alternate identity--in addition to the identity you used during activation. Today we allow you to add an alternate identity if you
used a Microsoft account to activate your subscription. This way you
can also add a work or school account (which you use when logging into
Visual Studio, Office 365, or your corporate or school network),
allowing you to access VSTS using both your personal account and your work or school account.
Detail steps please take a look at:How to add an alternate identity to your Visual Studio subscription
- If you can't sign in when choose both please see: Why can't I sign
in after I choose either "personal Microsoft account" or "work or
school account"?