Sonia, I think that this is an English computer manual type of sentence and it is really not very good English because sometimes they cut sentences down very much and really assassinate English. I am a computer science graduate, and my boyfriend (he is British) who knows even more than I about processes such as polling also thinks the same.

customize = personalizar

From The Free On-line Dictionary of Computing (09 FEB 02):

poll
To check the status of an input line, sensor, or memory location to see if a particular external event has been registered.

=======to tell you the truth although I know what polling is I don't know if there is a better term than the one I suggested (sondagem) but sondagem is the sense of what polling does.=========

If the original sentence is badly written as I suspect it is, my suggestion makes sense. Dependes a lot on the context you haven't got...
Of course, an alternative is that in context we don't know about, there is a command called "custom" in which case the translation would have to be a bit different.

Alterar o comando "custom" pode afectar....

But what is the custom command? Doesn't seem to make much sense, unless it is:

Alteração das preferências para este comando pode...

Good luck, this is tough