Can you request for expedited processing of a Canadian visitor visa?
There is no standard way to request expedited processing for visitor visas. You may of course include your circumstances in a letter of explanation which may or may not be considered when the immigration officer decides the priority of your file.
If a Canadian sponsor is available with urgent justifications, the sponsor can contact the office of their Member of Parliament for assistance. For certain serious humanitarian circumstances (e.g. accidental or sudden death/serious injury of a foreign national), sometimes the embassy or consulate of the foreign national's country (or their close family's country) can also be helpful.
Sometimes you do not need to, it just gets expedited. This is called the CAN+ program. It's automatic https://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=877&top=16
Visa application centres will screen all applications for CAN+ eligibility. Applications that show that the applicant travelled to Canada in the last 10 years or holds a valid U.S. non-immigrant visa will be sent to the visa office for expedited processing.
and requires no specific forms https://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=875&top=16
Is there a different application form for CAN+?
No. Simply apply for a visitor visa online or on paper.