Try bringing in the decimal field as varchar(50) and then using a derived column transformation to convert it rather than doing everything in the flat file connection manager. You should then be able to catch the error.

Or bring it to a staging table that is unconverted (all fields varchar for example) then convert it as a second stage which is what I do for a big daily import, combining unformated data from several flat files into each staging table.

If the conversion is then part of an input select statement eg select convert(decimal(9,9), myvalue) etc

then use a case statement to validate the number

e.g. select case when isnumeric(myvalue) then convert(decimal(9,9), myvalue) else null end....

and so on.

I found this is also useful with dates (using isdate())