How can I manually add a subscription to an account using Commerce Subscription Product module?
I've honestly only quickly glanced at it right now, but as far as I can tell, a valid subscription is simply the field field_commerce_sp_time having a value, in number of weeks the subscription is valid from today.

Thus, you should be able to simply edit the user entity as you usually would, and set any number of week you want. Note that the module ships with relevant permissions for editing the field, so you need to have the field_commerce_sp_validity permissions.

Similarly, you could directly insert the relevant value into the database with 

db_insert('field_commerce_sp_time', 'fcst')
  -&gt;fields(array(
    'entity_id' =&gt; $uid,
    'entity_type' =&gt; 'user',
    'value' =&gt; $number_of_weeks,
  ))
  -&gt;execute();