You can, and probably should, supplement your I-601 before it is decided by the USCIS. You should send the Hardship Letter and any other materials to the same USCIS office to which you submitted your I-601. Be sure to include your alien number and the receipt for your I-601.
For more information, please see http://shusterman.com/unlawfulpresencesimmigrationbars.html
If the I-601 was filed based on extreme hardship, it would be hard for an attorney to defend a decision submitting the form only, with no supporting evidence. Properly filed an I-601 has many letters from family, friends, medical professionals it can also have invoices, overdue bills, or evidence showing whatever you claim your hardship to be.
A bad waiver is filed with a letter from the qualifying relative stating "I would be devastated of my relative were not allowed to live here." A good waiver shows WHY you would be devastated, and contemplates life in the US with the relative versus life in the US without the relative versus life together in the foreign country.