You can use almost anything as a cake filling! The texture of the product is what will tell you what you have to do with it. If your preserves are too thick to spread, you might try warming them, yes. If they're too soft, you'll want to use a lot less than you would of a thicker filling. You COULD try mixing your preserves with buttercream for a lighter, less strong flavor. I like to do that if I'm using a fruit preserve, because I usually think they're a little too sticky/sweet on their own. Good luck! 
I haven't made this for a cake filling(its what I use for my fruit salad) but I think this would be a good cake filling.
1 can crushed pineapple(the big one, 15oz?)
1 pkg 4 serving size of french vanilla instant pudding
1 cup whipped cream
combine pudding mix to crushed pineapple, juice and all. Let set for 5 minutes. Fold in whipped cream. Spread onto cake(mix with fruit in my case).
I am going to be doing a mango filling and what I am doing is taking this recipie below from cake central and using mangos instead. Jen
Filling
2 pints stawberries (fresh only)
1/2 cup sugar
1 T fresh lemon juice
1 T Rum
2 T cornstarch
To MAKE FILLING:
Rinse, hull and slice the strawberries.
Place a quarter of the berries in a saucepan with the sugar and bring to boil. In a bowl, combine the lemon juice, kirsch and cornstarch and stir together till smooth.
Take the saucepan of berries off the heat and stir the cornstarch mixture into it.
Return to heat and bring back to a boil, stirring and cook for 2 minutes.
Remove from the heat, cool and then stir in the remaining sliced berries.
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