you can also use hittail.com if you already have a site that's getting traffic, and a nice free keyword generator that's good for longtails is Massive Keyword List Builder from ibizresearch.com
With excel you can put your base keyword in one column, then put your expansion list in the second column. For example: base keyword is in A1, expansion is B1, you'd put "=concatenate(A1, " ",B1)" in C1 That will combine them for you and voila, you've got a long tail keyword. Now repeat that for the entire list. You can also add in the different match types and other keyword variations and you can build a HUGE list.