Join another table to full outer join results from cte

Try ISNULL.

eg:

SELECT ISNULL(foo, bar)

will return bar if foo is null, and foo otherwise. So, in terms that might be a bit closer to your requirement:

SELECT ISNULL(b.empname, a.empname)

will return the empname field from table a if the empname field in b is NULL

Here's an example which re-worked your query using some sample data. I created temporary tables instead of using the table names in your CTE, but this should give you a pretty good idea of what's going on regardless.

create table #a
(
    tk_empl_uno int,
    month int,
    base_hrs int,
    base_amt int,
    time_class char(1)
);

create table #b 
(
    empl_uno int,
    period int,
    month int,
    billable_hrs int
);

create table #hbm_persnl 
(
    employee_code int,
    employee_name varchar(30),
    empl_uno int
);

insert into #hbm_persnl values(1, 'Bob', 1);
insert into #hbm_persnl values(2, 'Ted', 2);
insert into #hbm_persnl values(3, 'Sally', 3);
insert into #hbm_persnl values(4, 'Diane', 4);

insert into #a values(1, 1, 10, 10, 'T');
insert into #a values(2, 1, 40, 300, 'T');

insert into #b values (1, 1, 1, 150);
insert into #b values (3, 1, 1, 30);
insert into #b values (5, 1, 1, 60);

with A as 
(
    select 
       tk_empl_uno [empl_uno], 
       month, 
       sum(base_hrs)[Actual Hrs],
       sum(base_amt)[Actual Amt]
    from #a tah
    where
       tah.month = 1
    group by 
       tk_empl_uno, 
       month
),
B as 
(
    select 
       empl_uno,
       period,
       month, 
       bud.BILLABLE_HRS [Budget Hrs]
    from 
       #b bud
    WHERE
       bud.PERIOD = 1
)
select 
    p.employee_code, 
    p.employee_name,
    A.month,
    B.period,
    A.[actual hrs],
    A.[actual amt],
    B.[budget hrs]
from 
    #hbm_persnl p 
    /* This should be a left outer join.  See the comments below. */
    inner join A on p.empl_uno = a.empl_uno 
    full outer join B on b.empl_uno = p.empl_uno
where
    A.empl_uno IS NOT NULL or b.empl_uno IS NOT NULL
order by 
    b.period,
    p.employee_name;

The data set I ginned up will show two distinct types of "missing record" pieces of information. The first is if you have a record in B but not in A and the information exists in P. In that case, the query will end up returning NULL for the P information (employee code and employee name). The easiest way to correct this is to convert your inner join on A to a left outer join on A. In the example above, this is the record where the budget hours is 30.

The second type of "missing information" problem is when you have an Employee UNO in expression B but no record in table P (the personnel table). In that case, you have bad data and unless the employee information is stored somewhere else, you won't be able to display it. In the example above, this is the one where the budget hours is 60. There is no employee UNO of 5 in the personnel table, so we're out of luck trying to display that information.