I am the author of Ultisnips. This is hardly a place to ask specific UltiSnips questions because we will not see them. Next time, use the specific question / answer site that hosts the plugin: https://answers.launchpad.net/ultisnips
However, the function you are looking for is called a word extension. Use it like this:
snippet st "My cool st snippet" w stollen endsnippet
"w" will make sure your fragment expands in other words.
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