Is it possible to step up dc without changing it to ac or pulsed dc?
If your converter contains a transformer, or inductors, or capacitors, no. These components require some change to make them work to step up DC. We can use an output filter to remove as much of the pulsing as we like.
We usually put up with this pulsing then filtering approach, as we can get power efficiencies closely approaching 100%
There are much less efficient methods that can do the job under steady conditions.

A LED drives a number of series-connected PV (photo-voltaic) diodes.

This device is actually made and sold for the isolated powering of FET gate-drive circuits. It's hugely inefficient in terms of energy out for energy in, but for low power applications where noise and isolation are at a premium, they are fine. I did buy a little 300 mW solar panel and try driving it with a 10 W LED a while ago. They work together much as you'd expect.

An input Peltier, or a power resistor and heatsink combo, drives a number of series-connected Peltiers operating as TEGs (thermo-electric generators).

To the best of my knowledge, these are not sold commercially. I've often mused on this method for getting a small amount of quiet isolated power to an opto-isolated ADC converter, but then I lie down until the feeling passes. I believe the COP (coefficient of performance) for Peltiers is not too bad for small temperature differences, so you may get reasonable energy efficiency if the elements are under-run by a large fraction. At anywhere near their 'rated' power, efficiency would be awful.