Can 2 elements be combined into a new release without a "clan based Kekkai Genkai"?
Indeed, two or more elements can be mixed and utilized without it being clan based.
According to the Naruto wiki:

Wood Release (木遁, Mokuton, Viz: Wood Style) is a nature transformation kekkei genkai, a combination of Earth and Water Release. It was famously used by Hashirama Senju, and in fact is not known to have naturally occurred in anyone else, leading to it being regarded as impossible to use without having his cells.

Wood Release is a Kekkei Genkai only naturally used by Asura Otsutsuki and Hashirama Senju. Asura received Wood Release after Hagoromo Otsutsuki, the Sage of Six Paths, passed his power onto him. Wood Release is also of Otsutsuki lineage, another possibility as to why Asura has it. Hashirama most likely has it being his former reincarnate. This power is a mixture of two elements not used by any Senju except Hashirama.
The wiki also states:

A kekkei genkai (血継限界, literally meaning: Bloodline Limit) is an anomaly of the DNA that allows the wielder to use unique techniques. A kekkei genkai's name describes both the anomaly and the resulting technique.

You are not wrong on this part. But Kekkei Genkai are shown to not only be bloodline abilities. They are also known to be the mixture of two elements, not needing to be passed down. Examples of this are Hashirama, and Haku. Boruto may be able to make a new Kekkei Genkai, but as of now, it has not been shown.
Final Answer
Yes, it is true that Kekkei Genkai are typically known to be abilities passed down by family, examples being the Hyuuga's Byakugan, The Uchiha's Sharingan, and the Chinoike Clan's Ketsuryugan. But Kekkei Genkai are also known as the mixture of two elements.