Are you using Sharepoint Designer(SPD) to design these workflows?

If you are using SPD, I dont think you can get info like User's Manager
using the OOB features.
You can always implement a custom action in SPD.
This can be done by implementing a custom windows workflow  activity to
retrive the User's Manager from the Active directory(using
Activedirectory API ). This custom action can be used in SPD when
designing workflows.
Another option is to use the custom workflow activity to retrive the
manager info from the Sharepoint user profiles.

Even if you are using VisualStudio2005 workflows, you can use such
custom activity to get the manager info.

Microsoft has a lot of documentation about how you can create a workflow
activity.

I've been working on this myself, and I've had a hard time finding
anything comprehensive. However, there are some bits and pieces that
might help.
The first is, "So You Want To Develop sharepoint  Workflows..." by Eilene
Hao at Microsoft. It's available at:
blogs.msdn.com/sharepoint/attachment/1106451.ashx
MSDN magazine has had a few articles on workflow  that are somewhat more
tutorial-like, although they don't focus on SharePoint. Start with...

msdn.microsoft.com/.../MMCDi
splayFeed.asp?sort=pubdate&lang=en&product=103364&audience=100402
...and follow the links from there.
MSDN has a fairly high-level article:
msdn2.microsoft.com/en-us/library/aa830816.aspx
and another with links to a walkthrough (which itself is so-so):
msdn2.microsoft.com/en-us/library/ms549489.aspx
Saving the best for last, try these from Nick Swan's blog:
weblog.vb-tech.com/.../1753.aspx
weblog.vb-tech.com/.../1760.aspx