convert sql column from nvarchar to numeric

You can protect against conversion errors using iSNUMERIC() but this completely safe.

e.g.

select CONVERT( [as-am] as decimal(12,3) )
from that_table
where ISNUMERIC([as-am]) = 1

a result of 1 from ISNUMERIC() indicates the parameters holds numeric information but this can include items such as currency symbols

We could advise better if you supplied this:

select distinct [as-am]
from that_table
where ISNUMERIC([as-am]) = 0

How do you define the data type during import??  I do not see that option in the wizard

In the Import Wizard when you chose the worksheets and the destination table, click on "Edit Mappings..." button and will pop-up a new screen where you can change column names and data types.