Various solder metal supplies sell pre-forms as loose individual shapes, usually donuts. The loose performs then are placed on the connector leads. If you have any volume at all, you would want to look at a �shake table� that would vibrate a batch of performs until they fall into a machined recess in a plate having the connector lead pattern machined into it, slide the excess performs away and place the connector on top of the pre-forms. Another problem is getting the performs to stay on the contacts, so you have to ensure there is some resistance between the ID of the perform and the connector contact.
Solder arrays are also sold. These are donut like shapes of solder having a thinner ligature of solder connecting each shape so that you are only dealing with a single strip that is placed on the connector leads. The downside is that during reflow, you may experience solder �robbing� or �thieving� whereby one joint will have insufficient solder and the adjacent joint will have excessive solder.
Another alternative is buying the connector with solder and flux pre-applied to the leads, such as with solder-flux bearing leads.