How to transmit my PC onto my iMac? I use tiny 13'' Windows laptop for work and whenever working from home would like to use my 27'' iMac as a display to it. 

Is this possible? Would HDMI->Thunderbolt cable do the trick? Is there anything more required?
You cannot transmit the output of your Windows screen through an iMac display, but there is one way to work that you may find quite appealing: Microsoft Remote Desktop.

MS has a remote desktop client application that is a free download from the App Store. On your Windows machine, you change System preferences in Control Panel to enable remote access. In the Remote Desktop app on your iMac, you create a machine profile that specifies the Windows computer name or IP address, your account username and your password. It supports fullscreen mode, so your Windows login will be plenty big on your 27" display.

I use the application daily to remotely administer Windows desktops and servers all over the country. From the comfort of my MacBook.