SQL User Defined Function Within Select
Solution 1:
Yes, you can do almost that:
SELECT dbo.GetBusinessDays(a.opendate,a.closedate) as BusinessDays
FROM account a
WHERE...
Solution 2:
If it's a table-value function (returns a table set) you simply join it as a Table
this function generates one column table with all the values from passed comma-separated list
SELECT * FROM dbo.udf_generate_inlist_to_table('1,2,3,4')