The best way to add information that is not one of the common fields is to use the custom fields option available with the Plus version of MSP. If you are not using the Plus version, there is a work-around that can be used.
In some cases, data needs to be stored in MSP for which no field exists in a volunteer's profile, such as birthday's or dates when they began serving. What can be done is storing this information in the "Comments" field of a volunteer's profile. This data can be exported on rosters, as well as filtered throughout the program to send emails or build custom tokens for.
When importing volunteer data, you can also import information which does not fit in to a common field, directly to the Comments section for volunteers. From the File menu in MSP, select "Import volunteers..." In the Import Volunteers window, check the option below the Available Fields column that says, "Show all options" and you will then see the Comments field. Be sure to include this field in the lost of "Fields to import" so it matches up with the appropriate column on your spreadsheet, then all of the information in that column will be added to the appropriate volunteers' Comments section of their profile.
To filter by this information to send emails, in the Emailer pane, select the "Send to All Volunteers" option, and then check the "Moreover, only send to volunteer's passing this filter" box. This will open the "Filter volunteer list" window. On the lower right side, there is a section for "Comments" - check this option and you can set this to "Comments contain" to filter for anyone who has "January" listed in their profile to find January birthdays; or "2009" to find any volunteer who has been volunteering since 2009.