This* seems to work:

#! perl -slw
use strict;

local $/ = ''; # paragraph mode

while( my $clause = <DATA> ) {
    $clause =~ s[
        ( create \s+ table \s+ \S+ \s+ \( )
        ( .+ )
        ( \) \s* )$
    ]{
        my( $x, $y, $z ) = ( $1, $2, $3 );
        $y  =~ s[(?<!null)(,|\Z)][ not null$1 ]g;
        "$x$y$z";
    }esx;
    print $clause;
}

__DATA__
create table foo (
   somevar1 sometype,
   somevar2 sometype null,
   somevar3 sometype not null,
   somevar4 sometype(20),
   somevar5 sometype(20) null,
   somevar6 sometype(10))

create
table
foo
(
    somevar1
    sometype
    ,
    somevar2
    sometype
    null
    ,
    somevar3
    sometype
    not
    null,
    somevar4 sometype
    (20),
    somevar5 sometype(20)
    null, somevar6 sometype(10
    ))

create table foo ( somevar1 sometype, somevar2 sometype null, somevar3
sometype not null, somevar4 sometype(20), somevar5 sometype(20) null,
somevar6 sometype(10))
[download]

Produces

C:\test>junk3
create table foo (
   somevar1 sometype not null,
   somevar2 sometype null,
   somevar3 sometype not null,
   somevar4 sometype(20) not null,
   somevar5 sometype(20) null,
   somevar6 sometype(10) not null )


create
table
foo
(
    somevar1
    sometype
     not null,
    somevar2
    sometype
    null
     not null,
    somevar3
    sometype
    not
    null,
    somevar4 sometype
    (20) not null,
    somevar5 sometype(20)
    null, somevar6 sometype(10
    ) not null )


create table foo ( somevar1 sometype not null,  somevar2 sometype null
+, somevar3
sometype not null, somevar4 sometype(20) not null,  somevar5 sometype(
+20) null,
somevar6 sometype(10) not null )
[download]

*Note: I've use a simplified regex to demostrate the nested substitution technique. You will probably need to improve it match SQl syntax.