Yes you can but I would not start deconstructing the fireplaces or chimney without consulting a structural engineer.

We own a colonial which is a mish mash of buildings put together to make a house. Prior to renovating, the oldest part was from 1680 and the newest 1880. The house and rooms were very small. The house was condemned when we bought it. During rehab we decided rebuilding the multiple fireplaces and chimney was cost prohibitive so we tore down the chimney to the first floor and removed all the fireplaces except for the large kitchen fireplace which we kept for decoration only. We left the archway chimney base in the cellar as it supported some of the first floor.

Removing the chimney and fireplaces enabled us to enlarge the living room and bedrooms and add a pantry on the first floor and bedroom closets on the second floor.

We did the demolition ourselves but we consulted a structural engineer prior to starting. We ended up having to add a steel beam to hold up the second floor as a good bit of the second floor was held up by the chimney.

Bottom line, tread carefully.

If I were DIY using a chimney for something other than a chimney, I'd first make sure the top of the chimney is capped and well sealed from birds and squirrels and other critters and weather.

I'd also want to add insulation within the chimney above anything built in the firebox/niche .. so building a thin box as a liner for the niche might be the next step. Then what you build to slid into the niche could be better protected from soot/scratches, etc. -- and that is likely the way I'd build it: separate and able to slide it into the niche and remove it ... depending, of course, upon what fireplace you have and you want to build

A picture and measurements could help.

I have an old house c. 1905 which had 2 brick chimneys. The one in the kitchen was long since removed before we bought it (a portion of this chimney remains in the attic, and should be removed as it presents a seismic hazard), and the space filled in with a set of shelves which I use for cookbooks and spice rack. The one in the living room we completely removed ourselves, from basement through 1st and 2nd floors and roof, and patched the holes in the roof, ceiling and floor. We simply dry walled over the hole in the walls. Neither chimney was structural, and there was no fireplace with either one. Removing the chimney was just a lot of back breaking labor, but not complicated. Patching the hole in the roof and floor was the hardest part.

I would just remove it- one less spot for the roof to leak, the bedrooms gain a closet, and you have more space in the dining room.