Hello Dennis,

no, you can't. You will need a Sql Server out of the Microsoft Sql Server familiy. Please note that there exist free versions of that server (http://msdn.microsoft.com/sql/express/).

DotNetNuke has a  provider modell which would allow to write your own Data provider to store your data inside MSAccess/Xml or what evear you want. But it will make no sense because of missing features of that platform. Yes, Access accects SQL Commands, but it has no idea to handle stored procedures, DDL commands are limited. There are some projects writing providers for firebird and mysql, these platforms offer a lot of the needed features though this will last a couple of months before beeing usefull.  

Nevertheless you might be able to write your own Access provider, but be aware that this is not done in hours or days, it will take months or years. If you finally finish, this will result in poor performance and stability.