Does it not work for you to add this as a custom profile property?  This is the recommended mechanism for adding fields to users.  "Mauling" the DNN tables will only cause you more problems down the road.

Hope that helps,

Whoa!  Take a step back here.  Just because the generated ID is dynamic doesn't mean you can't or shouldn't use the custom profile properties table.  Take this example:

SELECT PropertyDefinitionId from ProfilePropertyDefinition where PropertyName = 'MyPropertyName' and PortalID = 0

This will get you the PropertyDefinitionId for you for a custom property.  Now lets take it a step further... perhaps you want to grab user information where they have a certain value for a certain property.  Try this (this is dynamically generated SQL I pulled from a module I wrote at work... it has a lot of junk in there but illustrates my point perfectly):

SELECT Users.UserID, Users.Username, Users.FirstName, 
Users.LastName, Users.IsSuperUser, Users.AffiliateId, 
Users.Email, Users.DisplayName, Users.UpdatePassword, 
UserPortals.PortalId, UserPortals.Authorised FROM Users 
LEFT OUTER JOIN UserPortals ON Users.UserID = UserPortals.UserId 
LEFT OUTER JOIN UserProfile AS UserProfile_Responsibilities 
ON Users.UserID = UserProfile_Responsibilities.UserID AND 
UserProfile_Responsibilities.PropertyDefinitionID = 
(SELECT PropertyDefinitionId from ProfilePropertyDefinition 
where PropertyName = 'Responsibilities' and PortalID = 0) 
LEFT OUTER JOIN UserProfile AS UserProfile_Other_Language 
ON Users.UserID = UserProfile_Other_Language.UserID 
AND UserProfile_Other_Language.PropertyDefinitionID = (SELECT 
PropertyDefinitionId from ProfilePropertyDefinition 
where PropertyName = 'Other_Language' and PortalID = 0) LEFT OUTER JOIN 
UserProfile AS UserProfile_Position ON 
Users.UserID = UserProfile_Position.UserID AND 
UserProfile_Position.PropertyDefinitionID = 
(SELECT PropertyDefinitionId from ProfilePropertyDefinition 
where PropertyName = 'Position' and PortalID = 0) 
LEFT OUTER JOIN UserProfile AS UserProfile_Position_Long ON 
Users.UserID = UserProfile_Position_Long.UserID AND 
UserProfile_Position_Long.PropertyDefinitionID = (SELECT 
PropertyDefinitionId from ProfilePropertyDefinition where 
PropertyName = 'Position_Long' and PortalID = 0) WHERE 
UserPortals.PortalId = @portalId1 AND UserPortals.Authorised = 
@approved1 AND (UserProfile_Responsibilities.PropertyValue 
LIKE @searchTerm1  OR UserProfile_Other_Language.PropertyValue 
LIKE @searchTerm1  OR UserProfile_Position.PropertyValue LIKE 
@searchTerm1  OR UserProfile_Position_Long.PropertyValue 
LIKE @searchTerm1) 

So one thing to note is that I'm using parameterized queries and, as mentioned, this is dynamic SQL so I built my filter "on the fly" based on user criteria entered in a search.

There is no reason not to use the custom profile fields section... it is quite useful and you don't have to worry about breaking upgradeability.

If you want information about how to write to those profile fields in your own code just post up and ask and besides myself several other developers can help steer you in the right direction there.