If someone finds it a good way to have a concentration in pray while on his knees, can it be done? (Not talking about Amida/Sidur prayers, but talking with the creator.)
Since praying on their knees is a distinct sign of Christianity (is it?), it is forbidden to resemble non-Jews in their conduct.
"אֵין הוֹלְכִין בְּחֻקּוֹת הָעוֹבְדֵי כּוֹכָבִים וְלֹא מִדַּמִּין לָהֶן לֹא בְּמַלְבּוּשׁ וְלֹא בְּשֵׂעָר וְכַיּוֹצֵא בָּהֶן"
"It is forbidden to follow the customs of the idolaters, or to imitate them neither in dress nor in hair-trimming and like customs, for it is said: "And ye shall not walk in the customs of the nation which I cast out before you" (Lev. 20.23), and it is also said: "And in their customs shall ye not walk" (Ibid. 18.3), and it is, moreover, said: Then take heed to thyself that thou be not snared by following them" (Deut. 12.30)—all of which pertain to one subject admonishing them not to be like unto them. Forsooth an Israelite shall be separated from them, and be recognized by his clothes and in his conduct as he is different than they are in education and tendencies. For even so it is said: "And have set you apart from the peoples, that ye should be Mine" (Lev. 20.26)."
PS: Thanks YaacovDeane: The practice of praying on the knees is mentioned in the Scriptures. BUT It is important to stress, that even the practices that were "originally" Jewish and later adopted by other religions are forbidden to follow