Can a dNTP be built into a RNA strand? DNA consists of deoxyribonucleotides, RNA consists of ribonucleotides. They differ mainly (apart from the uracil / thymine difference) in the sugar part, the deoxyribose and the ribose. Those two molecules differ in the hydroxy group in the ribose which is only a single proton in the deoxyribose. This part of the sugar molecule is not directly involved in binding reactions, nevertheless it causes the whole difference in RNA and DNA. I wonder: could a dNTP be used in an RNA strand (or vice versa)? Is it chemically possible that we have a RNA molecule that contains a dNTP next to its NTPs?

This is rather easy to do if you synthesize oligonucleotides chemically and not enzymatically. This is typically done using phosphoramidite chemistry, and it allows for the synthesis of chimeric RNA/DNA oligos. You can even incorporate modified nucleosides like 2'-O-Me or LNA.

This is typically done if you want to change the properties of an oligo, e.g. if you want to make it resistant to degradation by enzymes.