You should be able to. When you go to My CIC there should be an assessment to tell you if you're eligible. Obviously I can't log in for you, but create your access key if you don't have one, then log in. Once you're in, request to change status from visitor to TFW. The first thing you'll do is determine eligibility. Simce you were already admitted as a visitor, as long as you have the LMO, you should be fine.
If you do the online application, you'll need a scanner or very good camera to upload copies/pictures of the LMO and job offer letter. MyCIC sometimes is very busy, so use a desktop with a hard wired Internet connection if you can - as opposed to a laptop on wifi.
http://www.cic.gc.ca/english/e-services/mycic.asp?utm_source=immigration.gc.ca%2FMyCIC&utm_medium=URL_direct&utm_campaign=form-help