Maris - thanks for asking a question that taught me something new!
I have never seen nor heard of "sugar with pectin" ... aka: jelling sugar
. But it sounds interesting. From what little I was able to find out about it - it is a combination of sugar, pectin and citric acid blended "in the correct proportions" for a 40% fruit/60% sugar ratio ... or roughly for each 1 cup of fruit and juice you need 1.5 cups jelling sugar.
So, did you make your jelly something like this: First you processed your fruit in some manner to extract the juice and strained it ... the you brought the juice to a boil, added the sugar, stirred to dissolve it and brought it back to a boil for 1 minute - then ladled into sterile jars?
If so - using the jelling sugar - I would assume you could get good results making strawberry jam in a similar fashion. Mash the strawberries and measure ... put in a pot and bring to a boil stirring almost constantly, add the jelling sugar (1.5 cups per each 1 cup strawberries and juice), bring back to a boil and boil for 1 minute - then remove from the heat and ladel into your steralized jars.
Like I said - I've never seen this stuff ... I'm just trying to extrapolate a reasonable solution between what I could find on jelling sugar and the process using fruit, sugar, acid and powdered pectin.
I tend to go the route that doesn't use powdered or liquid pectins as much as possible ... it uses less sugar (not as sweet but more natural fruit flavor) but does take a lot more time (20-30 minutes cooking time) and effort- and a good candy thermometer, and patience.
For 6 cups mashed strawberries - in a heavy bottomed non-reactive pot add 5 cups plain sugar, stir to combine well, and then allow to macerate for an hour or so. Then, bring the mixture to a boil and cook until it reaches the jelling point (8ºF above the boiling point of water in your location - if your water boils at 212ºF then that would be 220ºF) on a candy thermometer (stirring very frequently) and add 2 Tablespoons bottled or fresh squeezed lemon juice. However - I have learned from experience that the temp isn't an absolute guarantee .. sometimes it needs to cook a little more to insure it jells. So, once it reaches "temp" I start testing it for the spoon/sheet test - and when it reaches that I double check it using the Refrigerator/Freezer Test. Here are instructions
for both of these. One thing missing in the "plate test" instructions is that you should put the plate in the freezer before you start cooking - you get quicker results.
As Bob Marley once said - "We're jamming, jamming - And I hope you like