SQL: How to ALTER TABLE with an aggregate function (derived, calculated fields)?

I have this assignment where I need to add columns to a table in the SQL Command Line Interface of Oracle 10g XE.

The database is for an air charter service and the table is CHARTER.

I need to add an attribute called CHAR_FLT_CHG_HR, which is copied from the MODEL table using an attribute called MOD_CHG_MILE.

I feel like it ought to be something like:

ALTER TABLE charter

ADD char_flt_chg_hr AS (

But that is as far as I get. How do I reference another table when I use the ALTER TABLE statement?

Further problems are as such:

CHAR_FLT_CHG is a new column that is char_hours_flown x char_flt_chg_hr, but I am unclear about how to use an aggregate function within an ALTER TABLE statement.

I can't test it out now because I don't have the char_flt_chg_hr column created yet, but it seems that it might be:

ALTER TABLE charter

ADD char_flt_chg AS (char_hours_flown x char_flt_chg_hr)

or something along those lines?

And further still, there is one that wants me to calculate a charge with tax, CHAR_TAX_CHG.

Would this be something like...?

ALTER TABLE charter

ADD char_tax_chg AS (char_flt_chg x .08)

.