If the source is a CD or any lossless encoding, it will be what it is - 16/44.1 in the case of the CD and likely the same for the lossless. If the source is a lossy compression format, like MP3, it has already been degraded and you have to decode it to PCM first and then convert the bit depth and/or resample.
Simply converting the bit depth does nothing. Resampling will create new samples that did not exist in the first place. If you resample you want to use an anti-alias filter and if the program allows it, set it to use the highest accuracy possible for the interpolation. You'll basically be making the original file larger with no increase in sound quality but at least it will be in the correct format for buring to dvd-audio.
If you can actually record the music originally at 24/96 then you can import it directly.