The Mac uses a different system for accessing additional characters from a keyboard layout. CrossOver supports the Mac keyboard input mechanisms, not the Windows one. So, entering Polish characters in CrossOver is just the same as entering them into any other Mac-native application, like TextEdit.
I recommend that you open System Preferences, select the International pane, and select the Input Menu tab. There, enable the Keyboard Viewer palette. Also, enable "Show input menu in menu bar". This will put the input menu, which looks like a national flag, in the menu bar near the right side. From that menu, select Show Keyboard Viewer. This will show you graphically which keys generate which characters in your current keyboard layout. If you hold down the modifiers (Shift, Option/Alt, etc.), the keyboard viewer will show you how those modifiers affect which characters are generated. If a key shows up in orange, it's a dead key -- it doesn't immediately generate any character but modifies the next key you type.
In the Polish keyboard layout, you can type "ą" by hitting the key just to the left of the Return key. On my U.S. keyboard, this key is labeled with the quote and double-quote characters. So, there's no need to use Option/Alt at all to generate that character. To type "Ą", you use the same key with the Option/Alt modifier.
In my testing, I typed all of the following in Notepad in CrossOver:
No modifiers:
<1234567890ż[
qwertzuiopó(;
asdfghjklłą
yxcvbnm.,-
Shift:
§%!?+=:_/"Ż]
QWERTZUIOPź)$
ASDFGHJKLŁę
YXCVBNMśńć
Option:
«&@#€`™¶•\„‚≠
Ū∑´®†°¨^£‘Ó{'
ūß∂Ů©ů∆۬…Ą
‹≤ˇ√ű~ŲųĢ—
Option-Shift:
»ĀāďĒēķ°–|”’÷
ĖėĮįĪīĶĻļ“Ź}~
ĽľŅņŐŌōŔŕťĘ
›≥Ŗ◊ŗÝýŚŃĆ
