For your first question, depends on which page you're talking about. For the Forms Generation Client event, you can. Once you have completed the wizard, you will find that the Temp web pages are created in the following directory:
<%YourVSDirectory%>CSWebsiteClientwebsite

You can open this site using VS (From File System) and change whatever is needed. Just make sure that you close the VS editing the web site so that it does not lock any files when you deploy your process.

Your second question can be addressed by using the SourceCode.Workflow.Client API. Samples, tutorials and API Members are available in the Documentation. You can also have a look at the following blackmarket project:
http://www.k2underground.com/k2/ProjectHome.aspx?ProjectID=22

Hi,

I have a process in which sharepoint client event is there. So a task is getting created for assigned user and he can action the task by editing the task. While editing, a page will be opened where he will be selecting one of the available actions to be performed. I want to edit the look and feel of the task edit page in sysn with the sharepoint site master page.

Regards

Ankur