Most of the Gram negative bacteria (and many Gram postives too) are able to bind and take in (transformation) and if sequences match, incorporate naked foreign DNA into their genomes. Different species are better at it than others. For example, members of the Neisseriacae (Neisseria meningitidis, Haemophilus influenzae) are naturally competent at taking up naked DNA and in Nmen in particular, globally, all strains are related. In the host mammal, evidence of incorporation of foreign DNA is shown by both vertical and horizontal gene transfer in many species.
Have K.pneumoniae and S.marcescens been well studied enough to say it is documented? - no, not to the same degree as Salmonella, Shigella, Psuedomonas aeroginosa and E.coli, all of which are much more pathogenic. Members of the family Enterobacteracae, to which Kpne and Smar belong, generally need to be induced in vitro to be competent (CaCl2 methods for example). Interestingly, E.coli is competent to take up DNA about 10% of the time naturally. Once the DNA is into the organism, it may be degraded by the organism's defences - restriction enzymes. It appears that Kpne and Smar are good at this.
Finally, incorporation into the genome requires some matching sequences between the naked DNA and the genome. In reality and the real environment, much of the transfer of DNA between bacteria is more likely done by phages and by conjugation. In vitro, methods include transformation, transduction (phages) and conjugation as well as electroporation (using an electrical shock). If you know all this, then this may be the only part of the answer for which you are truly looking!
REF: Microbiology. 2002 Apr;148(Pt 4):943-50. Development of a P1 phagemid system for the delivery of DNA into Gram-negative bacteria. Westwater C, Schofield DA, Schmidt MG, Norris JS, Dolan JW. Was not done with Serratia.