You can, but if you can't get the letter it's best to try to get letters from former managers or co-workers who no longer work with the company. Also, it's a good idea to have your original offer letter, check stubs, and tax documents (i.e. W-2 in the US) proving that you worked there. You can also go to the company website, and if the same title you held is listed, you can print out the job advertisement which will help to give them an idea of the duties of the position. Lastly, if you're not claiming points for that position, it's best to leave it off, and add it to the personal history section of your application. These items do not require a reference letter since you are not claiming points for them. Best of luck!