From what you have posted, I must assume that you are new to FileMaker and probably to relational database design.
This won't be as simple as you might think. The Time Billing and Contact Management tables serve the same basic function in their separate files. Linking these two files correctly, involves replacing one of these two with an external reference to the other--which can involve a lot of work updating calculations, layouts, scripts etc to work with then new data source table. It may be easier to take the contact management file and define the tables, layouts you need from scratch in this file to gain the features you want from Time Billing.
You could use import records with the New table option to import the Time Billing Line Items table into Contact Management and then link it to contact management by matching K_ID_Contact to K_ID_Employee. You'd then have layouts, scripts etc to create in contact managment to work with this.
Another option would be to examine the fields in the Time Billing and Contact Management and add any from Contact Mangement not already present to Time Billing, then import the Notes table and build the contact management features you like into Time Billing.
None of these options are what a person new to FileMaker is likely to find easy to do.