You could try old fashioned canning.
Otherwise, as Onyx said, roast or steam to make mash, then freeze. Roast pumpkin makes the best soup.

Pumpkin whoopie pies:
Pies:
275g plain flour
1/2tsp baking powder
1/2tsp baking soda
1 1/2tsp ground cinnamon
1/4tsp salt
200g light brown soft sugar (i use whatever i have)
125ml sunflower oil
1large egg beaten
1tsp vanilla extract
115g pumpkin puree

Filling:
200g full fat soft cheese (i use cream cheese or marscapone)
85g unsalted butter, softened
2tbsp maple syrup
1tsp grnd cinnamon
85g icing sugar, sifted

Bake:
Preheat oven to 180. Line 2-3 large baking sheets with baking paper.

Sift together plain flour, baking powder, soda, cinnamon & salt.

Place oil & sugar in a large bowl & beat for 1 minute with electric whisk. Whisk in egg & vanilla then pumpkin puree. Stir in the sifted flour mixture & beat until thoroughly incorporated.

Pipe or spoon 24 mounds of the mixture onto trays, spaced well apart to allow for spreading. (i never get this many. I get about 6 per tray, but i make them big. When it says they spread, they really do spread. Using an ice cream scoop can help)

Bake, one sheet at a time, in preheated oven for 8-10 minutes until risen & just firm to touch. Cool for 5 min then transfer to a cooling rack & leave to cool completely.

Filling:
Beat soft cheese & butter together until well blended. Beat in maple syrup, cinnamon & icing sugar until smoith.

To assemble, spread or pipe filling over flat side of half the cakes. Top with the rest of the cakes.

Recipe from: whoopie pies, by Love Food.

I've made these using left over apple puree instead of pumpkin. Very yummy