hook?

module/data/rollTable.mjs

@@ -0,0 +1,28 @@
+//Extra definitions for RollTable
+export default class DhRollTableData extends foundry.abstract.TypeDataModel {
+    static defineSchema(){
+        const fields = foundry.data.fields;
+
+        return{
+            formulaName: new fields.StringField({ // This is to give a name to go together with the core.formula
+              required:true,
+              nullable: false,
+              initial: 'Formula' // Should be a translation
+            }),
+            altFormula: new fields.ArrayField(
+                new fields.SchemaField({
+                    key: new fields.StringField({
+                        required:false,
+                        nullable:false,
+                        blank:true
+                    }),
+                    formula: new fields.StringField({
+                        required:true,
+                        blank:false,
+                        initial:"1d4" //Filler value
+                    })
+                })
+            )
+        }
+    }
+}