Auto Increment Field Based On Value Between Two Numbers

I have inherited an Access DB that is supposed to be kept "alive" but not made much better. On a weekly basis I get an Excel CSV that I'm supposed to import into one massive table; that works just fine.

However, there is another query that takes values from a table named "New Users From Import Table" and appends them to the current "User" table.

But before running that query I have to manually update a field called "Display Name ID" with the next highest number from the User table but only the highest value that isn't in the 9000 number range.

For example, my next highest "Display Name ID" is 1144, which of course does not include the records that range between 9000 - 9008.

I had to manually enter 89 "Display Name ID" values yesterday and I know there has to be a way to update that field when I pull the data over from the temp table without having to do it all manually.

Here's the query that pulls the data into the "User" table (after I've manually entered the incremented values).

INSERT INTO Users ( [Display Name ID], [User Type], Organization, [Display Name], [Alias Name] )
SELECT [New Users From Import Table].[Display Name ID], [New Users From Import Table].[User Type], [New Users From Import Table].Organization, [New Users From Import Table].[Display Name], [New Users From Import Table].[Alias Name]
FROM [New Users From Import Table];