>> You can add new fields only (probably, you can also change
>> name of existing fields but not datatype, length etc.).
>> If you want to change something, then you can 'Generate Script'
>> then edit it (basically it's CREATE TABLE... command),
>> drop existing table and generate new one.
>
>You meant there is no tool like "MODIFY STRUCTURE" that
>could automate the 3-step process in SQL Server's utilities?

AFAIK, yes.

>What would happen if I adjust the system catalogues for the
>tables? Will it allow me to do so?

I think it's highly risky way.