Based on the example (https://palantir.github.io/tslint/develop/custom-rules/) and totally untested. Also I don't know typescript.

maybe-like-this.ts

import * as ts from "typescript";
import * as Lint from "tslint";

export class Rule extends Lint.Rules.AbstractRule {
    public static FAILURE_STRING = "Prefer keyCode to key or code";

    public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
        return this.applyWithWalker(new PreferKeyCodeWalker(sourceFile, this.getOptions()));
    }
}

// The walker takes care of all the work.
class PreferKeyCodeWalker extends Lint.RuleWalker {
    public visitPropertyAccessExpression(node: ts.ImportDeclaration) {
        if (node.expression.text === 'KeyboardEvent' && node.name.text === 'code') {
            // create a failure at the current position
            this.addFailure(this.createFailure(node.getStart(), node.getWidth(), Rule.FAILURE_STRING));
        }

        // call the base version of this visitor to actually parse this node
        super.visitPropertyAccessExpression(node);
    }
}