how to design such a workflow in sharepoint designer.

Question

how can such a workflow be created in sharepoint designer.

should start when a new item is created in listA

Condition > if any item in the listB exist with customer name as andrews then

Action > update that item in listB with field colA = orignal value in the field + 1

Else

Action > Create item in list B with field colA = 1 and other fields whatever not important.

Answer 1

I am not sure I understand your questions completely, but if you have some C# or VB.net skills then you can achieve almost anything you like with the Workflow Power Pack (I worked on this), which allows these languages to be embedded in SharePoint Designer workflows without the need for Visual Studio.

Have a look here for more information.

Plenty of samples can be found here.

Answer 2

Apparently you are making a counter grouped by a common denominator (a username in this case). Thats a common scenario but hard to go in step by step, luckily Microsoft made an example of this in their Service Pack.