So custom fields are kind of the Achilles heal to Exchange Active Sync Accounts.
You see Exchange accounts push certain information like Calendars, Contacts, and Email to your devices. However in MS infinite wisdom when it comes to custom fields you cannot create them. MS gives you a few predetermined ones to use and that is it. MS 'solution' is the notes field.
I work in a corporate environment and I can tell you I run into this issue occasionally. I don't know why MS can't incorporate it but I can tell you they haven't.
Now your next question might be how come it works with my Android device? Well if you use gmail they in fact use EAS (begrudgingly) but also there own email pushing service (in conjunction with EAS) that isn't widely used that in fact allows custom fields.
I believe (and I could be wrong) the only way to add custom fields is to do so on a IMAP or POP3 account, manually on the phone again you might not be able to do this. If you can do this however you've lost the ability to get push services. However all the work you put into creating these contacts will be saved to skydrive and backed up so it's a one time pain in the a$$.
I hope I helped in some way.
By the way If i'm wrong about this please someone correct me. I'd love to know if this is possible.
-Chris