Is that possible to create a custom input widget in Foundry Slate?

You can't do this in a plain HTML widget - the dependency graph can't read state from arbitrary HTML input elements.

There are a couple other options here; the one that most directly works they way you would like it to in your question, would be to use the Code Sandbox widget, which effectively let's you build your own widget and wire it up to the Slate dependency graph for interaction with the rest of the app. You can use a 3rd-party library, assuming you have the license, to do something more advanced (you can search your Foundry instance for some examples in the Slate Reference Examples) or simply use HTML and Javascript to build the widget as you would in normal web development as you have access to the DOM and JQuery when working in the Sandbox.

Slightly more in-line with how Slate might expect you to build this functionality, you can use a single input widget, but toggle what you do with the associated input based on other state, for example what row the user has selected. This, in combination with a button that stores the input into a Variable with a click event, can be used to let users build up a "bag" of edits, that you can then apply with Actions (or you can apply them immediately - all depends on the workflow). You'll find some examples of that pattern in the "Events" folder in the Slate Reference Examples.

Thinking a bit more expansively, if you model your data in the ontology, you can set up an editable table pattern in Workshop and have a quite straightforward experience once you have the right Action and Object Type configuration. You'll find the documentation on this on your Foundry instance at https://www.palantir.com/docs/foundry/workshop/widgets-object-table/#inline-edits-or-cell-level-writeback and an example in the Flight Alert Inbox example application.