what about cheating a little and taking this approach (either way, i think negative look-ahead is going to help; see perlre)

$_ = do {local $/=undef; <DATA>};
s/ (sometype\(\d+\)|sometype)(?!.*?(not )?null\b.*?)/ $1 NOT NULL/mgi;
print;
__DATA__
create table foo (
   somevar1 sometype,
   somevar2 sometype null,
   somevar3 sometype not null,
   somevar4 sometype(20),
   somevar5 sometype(20) null,
   somevar6 sometype(10))
;
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